Switching power supply maintenance
Article Source：Jocol Group
Switching power supply in the industrial automation era, has been used to all industries, its precision circuit board and the strict requirements of the current power supply, making the switching powe...
Switching power supply in the industrial automation era, has been used to all industries, its precision circuit board and the strict requirements of the current power supply, making the switching power supply circuit board repair industry to become a relatively difficult one of the common fault fault equipment.
Before switching power supply maintenance, we must understand the working principle of switching power supply, the first high-voltage AC power through the full-bridge diode rectifier after a high voltage fluctuations in direct current, and then after the capacitor filter to become more smooth high-voltage direct current. At this time, the control circuit to control the high-power switch will be high-voltage direct current in accordance with a certain high frequency frequency sent to the high-frequency transformer primary. Then, the high-frequency low-voltage alternating current output from the secondary coil is converted into a direct current having a low voltage and a constant current capable of operating the load by rectifying the filter. Among them, the control circuit is an essential part. It can effectively monitor the output voltage value, and to the power switch signal to control the voltage up and down the amplitude of the adjustment. In the switching power supply, because the power input part of the work in the high voltage, high current state, the highest failure rate; followed by the output DC part of the rectifier diodes, protection diodes, high-power switching transistor more easily damaged, and then pulse width modulator Feedback and protection section.
One, in the case of power failure
First of all, before switching power supply is not energized, first with a multimeter to measure the voltage across the high-voltage capacitor first. If the switch is not switching power supply or switch open circuit caused by the failure, then in most cases, high voltage filter capacitor voltage is not released at both ends, the voltage of more than 300 volts, if you are not careful to touch, you must let You leave an unforgettable memory!
As the maintenance of power to be exposed to 220V high voltage, the body once the contact with more than 36V voltage is life-threatening. Therefore, in the possible conditions, as far as possible to check the power state in the absence of a clear short circuit, component damage failure. First, open the power of the shell, check the fuse is broken, and then observe the internal situation of the power supply, if the power of the PCB board components on the rupture, it should focus on checking this component, in general, this is the main reason for failure; Whether there is a paste inside, check whether there are charred components; ask the power after the damage, whether the power of the illegal operation, which is necessary for the maintenance of any equipment. After the initial inspection, but also on the power for more in-depth detection.
With a multimeter to measure the AC power cord at both ends of the positive and negative resistance and capacitor charging, if the resistance is too low, indicating a short circuit inside the power supply, the normal resistance should be able to reach more than 100 kΩ; capacitors should be able to charge and discharge, if damaged , The performance of AC power cord at both ends of the resistance is low, was short-circuit state, or it may be a switch breakdown. And then check the DC output part of the release from the load, respectively, the output of each group to measure the ground resistance, normal, the table should have a capacitor charge and discharge swing, the last indication should be the resistance of the road resistance. Otherwise the majority of rectifier diodes due to reverse breakdown.
Second, power detection
After passing the above test, you can carry out power test. This time is the key, need to have some experience, electronic foundation and maintenance skills. In general, should focus on checking the power input, switching transistor, power protection circuit and the power supply output voltage and so on. If the power supply is stopped, the power supply is protected and the voltage of the PWM chip protection input pin can be measured directly. If the voltage exceeds the specified value, it means that the protection of the power supply should be checked. Due to exposure to high voltage, it is recommended that no electronic basis friends need to be careful to operate.
Third, common faults
1. Fuse blown
Under normal circumstances, the fuse fuse shows the power of the internal lines in question. As the power supply in the high voltage, high current state, the grid voltage fluctuations, the surge will cause the power supply current instantaneous increase in the fuse fuse. Focus should check the power input side of the rectifier diodes, high voltage filter electrolytic capacitors, inverter power switch, check these components with or without breakdown, open circuit, damage and so on. If the fuse is indeed broken, you should first look at the various components on the circuit board to see if the appearance of these components have not been burned, there is no electrolyte overflow. If you do not find the above situation, then use a multimeter to measure the switch with or without a short circuit.
2. No DC voltage output or voltage output is unstable
If the fuse is intact, in the case of a load, the DC voltage at all levels without output. This situation is mainly caused by the following reasons: the power supply in the open circuit, short circuit phenomenon, overvoltage, overcurrent protection circuit failure, the oscillation circuit is not working, the power load is too heavy, high-frequency rectifier filter circuit rectifier diode breakdown, Filter capacitor leakage and so on. In the use of multimeter to measure secondary components, excluding the high-frequency rectifier diode breakdown, load short-circuit situation, if the output is zero, you can certainly be the power of the control circuit out of the fault.
3. Poor power load capacity
Poor power load capacity is a common fault, are generally appear in the old or long working hours of power, the main reason is the aging of the components, switch the work of instability, there is no timely cooling. Should focus on checking whether the regulator diode heat leakage, rectifier diode damage, high voltage filter capacitor damage.
Power supply maintenance example 1: a device power, no voltage after power output, the power inside the issue of 'creak' sound. This is a typical feature of power overload or no load. First carefully check the various components, focusing on the detection of rectifier diodes, switches and so on.
After careful examination, it was found that the surface of a rectifier diode (1N4007) had been burned and the circuit board was also burned. Find the same type of diode replaced, in a multimeter with a test is really breakdown. Connected to the power supply, the fan does not turn, 'squeaky' sound still exists. With a multimeter + 12V output only + 0.2V, +5 V only 0.1V. This indicates that the power supply is self-protected when the component is broken down. Measuring the primary and secondary switch tube, found that the primary switch tube has been damaged, with the same type of switch tube replacement, troubleshooting. Check the detours above, not before power, should measure the switch tube is damaged.
Power Maintenance Example 2: No 'creaky' sound, a fuse on a fuse burned. As the fuse is constantly being blown, the search range is narrowed. There may be three kinds of situations: rectifier bridge breakdown, large electrolytic capacitor breakdown, the primary switch tube breakdown. The rectifier bridge of the power supply is generally divided into four rectifier diodes, or the four diodes are cured together. It would be normal to remove the rectifier bridge. Large electrolytic capacitors removed after the test is also normal, pay attention to welding back to pay attention to positive and negative. The last may only switch off the tube. The power of the primary is only a high-power switch. Remove a volume of really breakdown, find the same type switch tube put on, the problem is solved.
In fact, the power supply is not difficult to repair power, the general power failure can be summed up as follows: fuse blown, rectifier diode damage, filter capacitor open or breakdown, switch breakdown and power supply protection, and other switching power supply circuit is relatively simple , The fault type is relatively small, it is easy to determine the fault location. So the power supply maintenance as long as there is sufficient basic knowledge of electronic, and then look at the relevant technical articles, usually more hands-on, pay attention to the accumulation of maintenance experience, power failure repair or very good solution.
Any chip-level maintenance work, are inseparable from the accumulation of experience and flexible maintenance ideas. In the event of failure can not be ruled out, or can not find the root causes of failure, often stop, or simply rest for a little while, multi-angle thinking and troubleshooting, often have unexpected results.